1. Field of the Invention
The present invention relates to a high-temperature gas sensor for the detection of the heat tone of combustible gases in the form of a semiconducting ceramic layer with thermistor properties. The present invention also relates to a process for the manufacture of the high-temperature gas sensor.
Known technical solutions start with resistance thermometers based on noble metals, such as Pd-Au alloys, as is disclosed in the paper entitled: "Trans. Faraday Soc." 62 (1966) 2566, or coils of thin Pt wire, as is disclosed in the paper entitled: "Solid State Gas Sensors", IOP-Publ. Ltd, England 1987, which were proposed in U.K. Patent GB 892 530 for the control of combustion processes. The comparatively weak relationship between the electrical resistance of such metals and temperature is utilized so that the reaction heat released on the surface of a heated resistance wire as a consequence of catalytic oxidation of reducing gases serves as a temperature signal.
Applications at up to 800.degree. C. have been suggested. Due to the relatively low sensitivity of only a few tenths of 1% change in resistance per degree on average, detection is severely and adversely affected by outside influences, such as the gas flow velocity. The detection range is thus restricted to approximately 0.1% of a combustible gas, as is reported in the paper entitled: "Measurement and Control" 22 (1989), 176. In order to increase the sensitivity and reduce the threshold temperature, German Published, Non-Prosecuted Application DE-OS 30 40 243 proposed the use of coatings with catalysts, such as aluminum oxide. The step to planar, integration-ready configurations for such gas sensors was documented in German Patent DE-PS 30 28 249.
Other technical solutions specify the use of a semiconducting component with a steep characteristic and therefore correspondingly greater sensitivity instead of the resistance wire. German Published, Non-Prosecuted Application DE-OS 38 39 414 A1 discloses that, in such a case, the temperature range is restricted to 200.degree.-500.degree. C., and German Published, Non-Prosecuted Application DE-OS 37 43 399 discloses that a passivating layer is required as an intermediate layer on the outside beneath the catalyst layer to protect the semiconductor. Metal oxide semiconductors, such as SnO.sub.2, ZnO, WO.sub.3 or In.sub.2 O.sub.3 with an amorphous catalytic outer layer are described in German Patent DE-PS 29 33 971.
Another variant of proposed technical solutions for the detection of calorimetric signals with sufficient sensitivity relates to sensors which utilize the pyro-electric effect, such as is disclosed in the paper entitled: "Ferroelectrics" 54 (1984) 211 or in U.S. Pat. No. 3,861,879, or that utilize the steep temperature-resistance characteristic of a PTC thermistor ceramic, such as are disclosed in the paper entitled: "Sensors & Actuators", B1 (1990), 54. In the former case, application is limited by an upper temperature range, the ferroelectric phase conversion point, and is limited to the narrow temperature range of the ferroelectric phase conversion in the latter. The long-term stability of such materials under the application conditions are particularly problematic.
Due to the high-degree of non-linearity of the resistance-temperature characteristic, temperature measurement sensitivity in the range of a few percent change in resistance per degree as well as a high degree of stability and reproducibility of the measurement of a calorimetric signal can be expected with NTC thermistor sintered ceramics, such as is disclosed in German Published, Non-Prosecuted Application DE-OS 42 13 629 for non-aging thermistors, such as in the systems Fe.sub.x Ni.sub.y Mn.sub.3-x-y O.sub.4 and Zn.sub.z Fe.sub.x-z NiMn.sub.2-x O.sub.4 (B constants 3400 to 4000 K), or as is disclosed in German Published, Non-Prosecuted Application DE-OS 42 13 629 for high-temperature applications up to 750.degree. C., such as MgNiMnO.sub.4 (B constant 4550 K), or as is disclosed in German Published, Non-Prosecuted Application DE-OS 43 27 285 for temperatures up to 1200.degree. C., such as Sr.sub.7 Mn.sub.4 O.sub.15 (B constant 10500 K).